1. Field of the Invention
The present invention relates to a printing head that adheres toner to a printing medium, and more particularly to an electrostatic ink jet printing head and a method for manufacturing it.
2. Description of the Related Art
One of the more popular printing methods recently is a non-impact printing method in which noise generated during printing is reduced to a negligible level. Ink jet printing has been deemed to be a particularly effective non-impact printing method and has a variety of advantages and effects as described below.
(1) The mechanism is simple and production cost is low. PA1 (2) Printing can be done directly on a printing medium at high speed. PA1 (3) A special printing medium is not necessary (plain paper can be used).
The above described ink jet printing method is preferable to other non-impact printing methods in view of ease in controlling the amount of ink. Therefore, it is anticipated that use of this printing method will increase.
Various kinds of ink jet printing methods have been propsed. For example, a method on the basis of heat, a method using a piezoelectric element, a method using an air flow, a method under the influence of an electrostatic force and so on have been proposed.
Ink jet printing methods depending on electrostatic force (an electrostatic ink jet printing method) are carried out by using ink having toner particles dispersed in a carrier liquid, applying voltage between needle shaped printing electrodes for ejecting the ink and counter electrodes provided on the rear surface of a printing sheet opposed to the needle shaped printing electrodes, and by propelling the ink under the influence of the electrostatic force of a generated electric field.
An example of an electrostatic ink jet printing head disclosed in Japanese Non-examined Patent Publication Sho 60-228162 will be described with reference to FIG. 6, which is a perspective view of the electrostatic ink jet printing head.
Referring to FIG. 6, a base plate 101 is made of a plate shaped insulating material. A plurality of printing electrodes 102 are formed at constant intervals with a desired resolution on the surface of the base plate 101. In addition, the printing electrodes 102 are so formed that the adjacent electrodes do not contact but are formed respectively as independent electrodes. An end of each printing electrode 102 is connected to a driver (not shown) applying respectively high voltage pulses to each electrode. The protrusions 103 are respectively formed at the ends of the printing electrodes 102. Ink ejecting openings 107 are formed slightly recessed from the protrusions 103. The cover 104 comprises an ink tank portion 105 so as to supply the ink to the ink ejecting openings 107. The supplied ink forms ink menisci at the end parts of the respective protrusions 103. Each of the independent electrodes 102 has one end at the side opposite to the ink ejecting side which is connected to the driver (not shown) as mentioned above. A high voltage pulse is selectively applied to the one end of each electrode from the driver in the course of a printing operation and a part of the ink meniscus is discharged or ejected by virtue of the electrostatic force, so that an ink droplet is propelled to a printing medium (not shown), fixed and recorded thereon.
However, the above described electrostatic ink jet printing head has the following problems.
Specifically, the ink located at the end parts of the protrusions 103 which serve as ink ejecting points is undesirably continued and connected together because of the presence of the ink menisci between the adjacent protrusions 103. As a result, a vibration generated on the liquid level of the ink meniscus at the ink ejecting point to which the high voltage pulse is applied and from which the ink droplet is ejected causes an adverse influence to be given to the ink at the end parts of the protrusions in the neighborhood thereof. Thus, ink droplets have been erroneously and disadvantageously ejected also from parts near to ink ejecting points where the ink droplets do not need to be ejected or forced out. Accordingly, the ink droplets have been unstably ejected. When the ink droplets are ejected irregularly, unnecessary ink droplets are propelled onto the printing medium and the contours of characters or figures to be printed have poor resolution.
Further, since the end parts of the protrusions 103 which act as the ink ejecting points are formed at positions retracted from the end part E of the base plate, there is a risk that the ink menisci are not formed in definitely or clearly protruding shapes at the ink ejecting points owing to the wetness of the end part E of the baseplate by the ink. When the ink menisci are not created in the protruding or convexed forms, an electric field generated by applying the high voltage pulse for ejecting the ink droplets has not been concentrated onto the end parts of the protruding or convexed shapes of the ink menisci, and therefore the ink has not been stably ejected or propelled. Consequently, there has also arisen a problem in that the ink droplets are ejected from parts other than the prescribed ink ejecting parts. In this case, the ink droplets have also been unstably ejected.
As stated above, according to the conventional electrostatic ink jet printing head, the unstable discharge or ejection of the ink, which results from the structure of the head and its ink ejecting points, has been generated, so that a beautiful and clear printing cannot be always carried out conveniently. Therefore, it has been desired to provide an electrostatic ink jet printing head capable of more stably ejecting ink.